1. Field of the Invention
The present invention relates to a bias current control method and related driving circuit, and more particularly, to a bias current control method capable of reducing power consumption and enhancing slew rate for realizing optimal system performance and related driving circuit.
2. Description of the Prior Art
An operational amplifier is a widely used element for realizing a variety of circuit functions. Take driving circuits of a liquid crystal display (LCD) for example, the operational amplifier can be used as an output buffer, which charges or discharges loading ends, i.e. liquid crystals, according to analog signals outputted by a front stage digital-to-analog converter, for driving corresponding pixel units on the LCD. However, with increases in size and resolution of the LCD, data quantity processed by the driving circuits per unit of time is also increasing significantly, so that response speed of the operational amplifier, also called slew rate, has to be enhanced as well.
The operational amplifier generally has a two-stage structure, which includes an input stage and an output stage circuit. The input stage circuit is utilized for increasing gain of the operational amplifier, while the output stage circuit is utilized for driving capacitive or resistive loads connected to the operational amplifier. In addition, since the operational amplifier may suffer loop instability problems, Miller compensation capacitors are commonly implemented to perform frequency compensation for improving loop stability. In the prior art, when the operational amplifier drives a load, the slew rate is often restricted by the bias current of the input stage circuit. In detail, the response speed (slew rate) of the operational amplifier is decided by the bias current of the input stage circuit and the driving capability of the output stage circuit. In this situation, the response speed (slew rate) of the operational amplifier can be expressed by the following slew rate equation:
      SR    =                  I        C            =                        Δ          ⁢                                          ⁢          V                t              ,in which “I” indicates a bias current, “C” indicates capacitance of the internal capacitors, and “ΔV” indicates voltage variation of the output voltage of the operational amplifier. This means that the response speed of the operational amplifier is decided by the charging (or discharging) speed when the internal capacitor of the operational amplifier is charged (or discharged) by the bias current of the input stage circuit. As can be seen from the above, when the bias current of the input stage circuit increases, the internal capacitors can be charged or discharged much faster, so that the response speed of the operational amplifier can be enhanced as well. In short, the internal slew rate of the operational amplifier is generally enhanced by increasing the bias current of the input stage circuit so as to increase the driving speed of the operational amplifier.
For example, please refer to FIG. 1, which is a schematic diagram of a driving circuit 10 with slew rate enhancement function according to the prior art. The driving circuit 10 includes an operational amplifier 102 and a bias current control unit 104. The operational amplifier 102 is utilized for generating an output voltage VO to a load LOAD according to an input voltage VI. The bias current control unit 104 is utilized for controlling a bias current of the operational amplifier 102 to enhance internal slew rate of the operational amplifier 102. Generally, the prior art increase a bias current I of the operational amplifier 102 by the bias current control unit 104 to enhance the internal slew rate of the operational amplifier 102 before the operational amplifier 102 begins to drive the load LOAD. Thus, the operational amplifier 102 would have enough driving capability to drive the load LOAD, i.e. charge (or discharge) the load LOAD. Please refer to FIG. 2. FIG. 2 is a schematic diagram of related signal waveforms of the driving circuit 10 shown in FIG. 1. A load input signal LD is utilized for indicating when to driving the load LOAD. Suppose the operational amplifier 102 will begin to charge or discharge the load LOAD on the falling edge of the load input signal LD. In this situation, the slew rate enhancement process should be finished before the falling edge of the load input signal LD. As shown in FIG. 2, during the period T1, the bias current control unit 104 controls the bias current of the operational amplifier 102 to keep at a high current level for increasing the slew rate. During period T2, since the slew rate has been enhanced, the bias current control unit 104 further controls the bias current of the operational amplifier 102 to keep at a normal operation level. During the period T3, the output voltage VO rises to a driving level shown in FIG. 2. However, during the period T2, i.e. from the time the required slew rate of the operational amplifier 102 has been enhanced until the operational amplifier 102 begins driving the load LOAD, since the operational amplifier 102 does not need to perform any operation, the provided bias current may cause additional power consumption.